Method of applying opaque dental ceramic material to a metal structure

ABSTRACT

A method of building a dental appliance and a kit for use in the method are provided. In the method, metal copings are coated with an opaque ceramic paste, and a coarse ceramic powder, having structural stability under the conditions needed to fire the ceramic paste, is dusted on the coating of ceramic paste, and the coping is fired. A coated metal coping with a rough, sandpaper like surface is produced, and subsequent layers of ceramic paste are applied to the rough surface to build up the dental appliance. The kit of the invention provides at least one preformed opaque ceramic paste, dusting powders, and other ingredients needed to build a dental appliance.

BACKGROUND OF THE INVENTION

This is a continuation-in-part of pending patent application Ser. No.018,211, filed Feb. 14, 1987 now U.S. Pat. No. 4,806,383.

An opaque dental ceramic (porcelain) paste for applying to dentalsubstructures, to be used in the preparation of ceramic dentalappliances, such as crowns and bridges, and a method of using the sameare provided.

When crowns, bridges and other metal dental substructures (copings) areto be veneered, the dental technician must first apply and fire a layerof opaque ceramic paste. This layer ensures a good bond to the veneeringceramic and, in addition, masks the unfavorable metal color of thesubstructure which is necessary to obtain a good aesthetic appearance.

Before the application of the opaque paste, an opaque slurry mayoptionally be fired on. For this purpose a thin suspension of ceramicopaque is mixed, applied and fired.

Conventional opaque ceramic paste consists of a pulverized opaqueceramic powder which is mixed with water, or a special modeling liquid,by the dental technician. The opaque powder itself consists of groundglass frits, which, owing to their chemical compositions can be meltedat temperatures below 1000° C. and opacifying agents. During firing,leucite is partially crystallized out, thus the thermal expansion of theglass is adapted to the thermal expansion of the metal. A high portionof opacifiers ensures a good masking of the dark metal. The opaquematerials are sold in several shades since they form the color basis forthe desired tooth shade.

In preparing the opaque ceramic paste, the dental technician must adjustthe consistency of the paste to obtain a paste which is sufficientlyviscous so as to adhere to the metal substructure without sagging, butnot so viscous that it cannot be easily applied. This procedure involvesmuch trial and error and is very time consuming.

Conventional opaque application requires much time and skill because avery even layer thickness has to be achieved. If the layer of ceramicopaque is too thin, the metal substructure shines through, when theopaque is too thick, there is not enough space left for the fullapplication of the subsequent body and enamel layers, which results in adiminished aesthetic affect.

Accordingly, there is a need in the art for an opaque paste, and amethod, which makes it possible to avoid the time consuming procedure ofmixing the paste in the laboratory, and makes it possible to applyopaque ceramic paste more easily and more consistently, using fewersteps, to dental substructures and whereby an even layer thickness isachieved.

SUMMARY OF THE INVENTION

The present invention provides a method for coating a metal substructurefor a dental appliance which comprises applying an opaque ceramic pasteto the metal substructure, dusting the applied opaque paste with acoarse ceramic powder having an average grain size of from about 40 umto 100 um, firing the opaque, and applying additional ceramic and firingas needed. The ceramic powder used for the dusting must have a thermalstability such that grain structure is maintained under the conditionsof firing. The method may comprise the addition step of using apreformed paste made by mixing a ceramic powder with an organic liquidsuch as glycerol. At least one additional layer of ceramic paste maythen be added to the fired opaque, and said additional layer mayoptionally be applied using a paste made by mixing a ceramic powder withan organic liquid. Preferably the ceramic paste will be pre-formed andwill be loaded into a package in which it can be stored, and from whichit can be easily dispensed.

A kit is also provided which contains the preformed, ready to use opaquepaste, the coarse ceramic powder, and optionally other ceramic materialsthat are needed to complete the building of a dental appliance.

Using the method, preformed paste, and the kit of the invention, it ispossible for the dental technician to build a dental appliance such acrown or a bridge using fewer steps, less time and less tedious methods.The preformed paste provided eliminates the need for time consumingtrial and error mixing of a paste in the laboratory. The assured qualitycontrol of the paste thaat is used, and the method in which it is usedlessens the possibility of applying an opaque layer that is too thin ortoo thick.

THE PRIOR ART

U.S. Pat. No. 4,064,311, to McLean et al., teaches a process for aceramic article which comprises firing porcelain onto a metal substrate.The metal substrate has an adherent layer of metal oxide depositedthereon which is wetted by the porcelain in the fused state.

U.S. Pat. No. 4,557,691, to Martin et al., teaches an opaque porcelainpaste comprising an opaque porcelain powder mixed with an aqueouscolloidal dispersion of urethan polymer. The paste can be applied as thefirst layer to a dental coping, and does not need to be fired prior tothe addition of the body porcelain layers.

European Patent Application No. 0,119,062 teaches a storable paste-likematerial for use in preparing porcelain dental prostheses comprising aporcelain powder, water, and a small amount of finely divided ceramicmaterial that overcomes the natural separation of the mixture ofporcelain powder and water.

DETAILED DESCRIPTION OF THE INVENTION

The ceramic powders used in the present invention broadly constitute aknown class of compositions. Conventionally, the powders used in makingdental ceramic pastes have a grain size below about 60 um. In theceramic powder used in the paste of the present invention, grain size iskept below about 50 um and preferably below about 40 um and in addition,powders of a grain size less than about 2 um are mixed therewith.Powders having an average grain size in the range of about 40 to 100 umare used in a dusting step.

In the method of the invention, either the powder used to make theopaque ceramic paste is ground to a finer grain size than usual, or aportion of very fine powder having a grain size below about 2 um isadded to the opaque ceramic powder, such that between about 10% and 20%of the ceramic powder comprises a ceramic powder having a grain sizebelow about 2 um. In the case where very fine powder is added to theopaque ceramic powder, the fine powder added can be a very fine opaquefrit or an opacifier such as CeO₂, ZrSiO₄, SnO₂, ZrO₂, or TiO₂ ormixtures thereof. As these opacifiers are supplied by the manufacturersin very small grain size, they need not be ground again.

In the method of the invention, the opaque powder is mixed with anorganic liquid having a boiling point below 300° C. to form a paste. Theorganic liquid used should not extend the firing times required andshould not generate any hazardous gases during firing. Organic liquidsthat may be used in the method of the invention may be selected from theglycerols, glycols, derivatives thereof, or dimethylsulfoxide.Particularly preferred is glycerol. Additives that increase theviscosity of the paste, such as alginates and stabilizing salts can beused. The consistency of the paste must be adjusted so that it can beapplied to a metal substructure in an even layer with one stroke of thebrush.

Using the paste of the invention as described eliminates the need formultiple application of the opaque slurry and firing steps as describedin the prior art.

For convenience, it is preferred that the opaque ceramic paste bepreformed and packaged in tubes or syringes so that required quantitiescan be easily extruded by the technician. Pre-forming the paste at thefactory ensures that a paste having a specific viscosity and compositionfor the purpose it is to be used is consistently produced, thus avoidingthe time consuming preparation of the paste in the laboratory. Theaddition of or presence of the fine grain particles, i.e. below 2 um indiameter, helps prevent separation of the organic liquid from the dentalpowder in the paste while it is being stored and improves thespreadability of the paste.

After a layer of paste has been applied, the coated substructure isdusted with a layer of coarse ceramic powder, having an average grainsize of between about 40 um and 100 um. In the dusting step, the coarseceramic powder is taken up with a wide, dry brush. The brush is tappedabove the object and the coarse grains of powder are thus released ontothe freshly applied paste. Other means of providing a dusting will beapparent to those skilled in the art.

The coarse ceramic powder used preferably is chosen such that it hasthermal stability under the conditions required for firing the ceramicpaste composition. That is, the coarse grains will have properties suchthat they will form a strong sintered bond with the ceramic paste whenthe substructure is fired, but the grains will not be significantlymelted or distorted by the firing so that a coarse grainy surfacesimilar to that of sandpaper will be obtained.

Alternatively, when extra large grains of powder are used in the dustingstep, ceramic powders may be chosen having a thermal stability such thata substantial portion of the grain structure is retained during firing,so that the sandpaper consistency of the surface, described above, isobtained.

A second layer of ceramic paste may then be applied to the substructure,filling the gaps between the protrusions of the coarse grains on thesurface of the ceramic opaque, and fired.

Also, the second layer oof ceramic, prior to firing, may be stained, toimitate anomalies in teeth. As is known to those skilled in the art,natural teeth are not perfectly smooth and do not have perfectconsistency of color. Accordingly, when artificial teeth are made havingperfect consistency, they do not look natural. To avoid this problem,and to provide a natural appearance, artificial teeth, after they arefired, are often coated with a stain paste comprising a find groundglass flux and pigments and fired again. During firing the stain pastefuses to the ceramic to provide a stained surface fused to the ceramic.In prior art methods of applying opaque, a first firing is requiredbefore the application of stain paste, apparently because the unfiredceramic is not stable enough to accommodate the "wet" stain paste. Usingthe ceramic paste and method of the invention, the stain paste may beapplied to the second layer of ceramic before firing, reducing furtherthe number of steps required to produce a finished product. As is knownto those skilled in the art, the stain paste can be prepared by mixing adry preparation of stain powder (fine ground glass flux and pigments)with a modeling liquid. According to the present invention, the modelingliquid may be an organic liquid having a boiling point less than 300° C.such as glycerol, glycol, derivatives thereof, or dimethylsulfoxide.According to the present invention, the stains can be blended with theunfired layer of ceramic before firing, if necessary.

Alternatively or additionally, the pigments may be printed onto asubstrate, parchment or flammable polymer for example, and placed on theceramic before firing. During firing the substrate burns off orevaporates leaving the pigments fused to the ceramic.

Accordingly, stains can be applied to the dental appliance without theneed for an additional firing.

As is known to those skilled in the art, the first paste layer used maybe made especially opaque, in order to completely mask the metal whichit covers, and formulated to form a good bond with the metal. The secondlayer may be shaded according to the desired tooth shade. Dentin andenamel layers may be added in due course.

The coarse ceramic grains distributed on the first layer of ceramicopaque simplifies the addition of the second layer of ceramic pastesince the technician need only concentrate on filling the gaps betweenthe grains to achieve an even layer. Further, like bricks in a wall thatoverlap end to end, the grains serve as posts or anchors to improve thebonding of the next layer of ceramic.

In the second layer of opaque paste, which is applied to the spacesbetween the coarse grains, the use of additional opacifying oxides suchas TiO₂ or SnO₂ in the composition is not desirable because of theirhigh opacifying effect. To prevent separation of the dental powder fromthe modeling liquid in the paste used in the second layer, it ispreferred that about 3% of a hydrophobic aerosil (such as Degussa R 972)be added to the organic liquid (preferably glycerol). The spreadabilityof this paste is not as good as that of the first layer because of theabsence of the fine (less than 2 um) particles, but high spreadabilityof the second layer is not needed because application of the secondlayer is made easy because of the presence of the coarse grains fired onto the first layer of paste.

The rough surface, resulting from the use of the coarse ceramicparticles, can also be used in the case where a silane bonding agent isused for applying and polymerizing an acrylic resin veneer. The presentmethod can also be used in the adhesive bonding technique. Both methodsare well known in the art.

The method of the present invention has the advantages that it saves thedental technician considerable time, because application of thedifferent layers of paste is made easier. There is less waste ofmaterial because the technique is simplified and the time spent on trialand error mixing of the paste in the laboratory is substantiallyeliminated. The opaque layers may be made especially thin and even, thusleaving enough room for body and enamel layers so that a good aestheticresult can be obtained. Also, since the technique is made simple, andmore even layers are obtained, the possibility of faults developing isreduced.

As an example of the composition that may be used for the first opaquelayer, the following is provided:

EXAMPLE

Composition of the opaque for the first layer:

    ______________________________________                                               SiO.sub.2   42    weight-%                                                    Al.sub.2 O.sub.3                                                                          12    weight-%                                                    K.sub.2 O   8     weight-%                                                    Na.sub.2 O  4     weight-%                                                    Li.sub.2 O  0.5   weight-%                                                    CaO         1.5   weight-%                                                    B.sub.2 O.sub.3                                                                           1     weight-%                                                    ZrO.sub.2   1     weight-%                                                    SnO.sub.2   10    weight-%                                                    TiO.sub.2   20    weight-%                                             ______________________________________                                    

All oxides are ingredients of a ground glass frit. Only the TiO₂ isadmixed to insure the presence of a fine powder, which is needed toinsure the good spreadability of the paste.

The composition of the opaque paste for the second layer will differslightly from the above formula because part of the opacifiers will bereplaced by pigments in order to produce the desired tooth shade.

The opaque, not including the TiO₂, has the following grain sizedistribution:

    ______________________________________                                        <5        <12       <24       <40     um                                      ______________________________________                                        21        66        98        100     %                                       ______________________________________                                    

On the average, TiO₂ has a grain size less than about 1 um.

The paste has the following composition:

    ______________________________________                                        opaque powder 67.7           weight-%                                         glycerol      32.0           weight-%                                         alum or other 0.3            weight-%                                         stabilizing salt                                                              ______________________________________                                    

The coarseceramic powder distributed on the opaque paste after it isapplied to the dental appliance substructure has the followingcomposition:

    ______________________________________                                        SiO.sub.2  56              weight-%                                           Al.sub.2 O.sub.3                                                                         13              weight-%                                           K.sub.2 O  9               weight-%                                           Na.sub.2 O 4               weight-%                                           CeO.sub.2  18              weight-%                                           ______________________________________                                    

The sieve fraction used: <80 um >40 um

The paste and dusting powder described in the Example will be fired atabout 980° C.

For convenience, the materials used in the method of the invention maybe provided in kit form.

The preformed opaque paste, with known firing properties, may bepackaged in a kit with a coarse ceramic powder that has the thermalstability required by the present method under the firing conditionsrequired to fire the pre-formed paste. If more than one ceramic pastehaving different properties for different applications, and havingdifferent firing requirements, and more than one coarse ceramic powderare provided in the same kit, the packages containing them can be colorcoded so that the correct coarse ceramic powder is used with the correctceramic paste. As will be apparent to those skilled in the art, manyspecific ceramic compositions may be matched with many specific dustingpowders by comparing physical properties that are known in the art.

In addition to the opaque paste, the kit may be provided with additionalceramics for use in the body layers and the dentin and enamel layers andother conventional materials as required. To the degree possible, thesematerials may be provided in a preformed paste, or in conventionalstates. For example, as described above, a paste composition of thesecond opaque layer may be provided.

While specific embodiments of the invention have been described herein,it will be apparent to those skilled in the art that the presentinvention may be modified and variously practiced without going outsidethe scope of the following claims.

What is claimed is:
 1. A method for coating a metal structure for adental appliance consisting essentially of the steps of: preparing anopaque ceramic paste by mixing an opaque ceramic powder with an organicliquid having a boiling point below about 300° C., coating said metalstructure with said opaque ceramic paste to prepare a coating of opaqueceramic paste on said metal structure, dusing said coating with a coarseceramic powder having an average grain size of from about 40 um to about100 um said powder having thermal stability such that at least asubstantial part of grain structure of said powder is maintained underconditions of firing said dental appliance, applying at least oneadditional layer of ceramic opaque paste, applying a stain paste to anadditional layer of ceramic opaque paste, and firing said dentalappliance.
 2. The method according to claim 1 in which said organicliquid is selected from the group consisting of glycerol, glycol, ordimethylsulfoxide.
 3. A method according to claim 2 in which saidorganic liquid is glycerol.
 4. The method according to claim 1 in whichsaid ceramic paste further contains about 10% to 20% by weight SnO₂,CeO₂, TiO₂, ArO₂, ZrSiO₄ or mixtures thereof, having a particle sizeless than about 2 um.
 5. A method according to claim 1 in which saidceramic powder used in said first opaque paste comprises grains having aparticle diameter of less than about 40 um.
 6. The method according toclaim 1 which comprises the further step of preparing the ceramic opaquepaste used for at least one of said additional layers of ceramic bymixing a dental ceramic powder with an organic liquid having a boilingpoint below about 300° C.
 7. The method according to claim 1 whichcomprises the further step of loading said ceramic paste into a packagein which it can be stored and from which said paste can be put directlyinto use.
 8. The method according to claim 7 in which said package is asyringe.
 9. The method according to claim 1 which comprises the step ofapplying said additional layer of opaque paste in such a manner that thegaps between projecting grains of coarse ceramic material are totallyfilled.
 10. The method according to claim 1 which comprises the step ofpreparing a stain paste by mixing a dry preparation of stain powder withan organic liquid having a boiling point below about 300° C.
 11. Themethod according to claim 1 which comprises the step of preparing astain paste by mixing a dry preparation of stain powder with an organicliquid selected from the group consisting of glycerol, glycol, ordimethylsulfoxide.
 12. The method according to claim 1 in which saidfiring is a vacuum firing.
 13. A kit for preparing a dental appliancecomprising:a first component (a) at least one pre-formed, shelf stable,ready to use opaque ceramic paste prepared by mixing an opaque ceramicpowder comprising a mixture of SiO₂, Al₂ O₃, K₂ O, Na₂ O, Li₂ O, CaO, B₂O₃, ZrO₂ and SnO₂ having a grain size iof about 5-40 um with an organicliquid selected from the group consisting of glycerol, glycol ordimethylsulfoxide, wherein said ceramic paste further contains about 10%to 20% by weight SnO₂, CeO₂, TiO₂, ZrO₂, ZrSiO₄ or mixtures thereof,having a particle size of less than about 2 um and a second component(b) a coarse ceramic powder comprising SiO₂, Al₂ O₃, K₂ O, Na₂ O andCeO₂ having an average grain size of from about 40 um to 100 um, inwhich said coarse ceramic powder has a thermal stability such that atleast a portion of said grain structure is maintained under theconditions of firing.
 14. The kit according to claim 13 in which saidpre-formed opaque paste component (a) is packaged in a syringe for easydispensing of said paste.
 15. The kit according to claim 13 in whichsaid organic liquid is glycerol.